U.S. Pat. No. 2,715,359 describes a horizontal sash type laboratory fume hood with a room air vent therebelow. A vertically slidable damper is actuated by horizontal motion of the sash by means of a sprocket, chain, and counter-balancing weight system, as shown in FIG. 4 of this patent. Such complicated mechanical linkage is cumbersome and expensive to manufacture and maintain. A slippage in a sprocket cog can cause the damper to be unbalanced and thus bind in its vertical track.
U.S. Pat. No. 3,604,333 describes a laboratory fume hood with a horizontally sliding sash, which in effect has an upper extension of the sash that opens an auxiliary air channel to the room as the sash is opened. Conversely, as the sash is closed, the auxiliary air passage to the room is shut off. The damper of this patent is limited in its usefulness to the very special purpose of controlling auxiliary air flow into the room because it operates directly with the sash; i.e., opens when the sash is open, and closes when the sash is closed. Such damper system would be inoperative to increase room air flow through a vent as a horizontal sash was closed, and decrease room air flow through such vent as the horizontal sash was opened. In U.S. Pat. No. 4,142,458, a horizontally sliding panel 18 has been connected to a conventional, vertical, sash type fume hood. However, the vertical movement of the main vertical sash of this patent opens and closes the room air vent, as is done in other conventional, vertical, sash type fume hoods.